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Garbage Collectors

Garbage Collection / February 19, 2017

We received some questions around the G1 garbage collector
and the use of Permanent Generation with it. There seems to be some confusion
that the Hotspot JVM does not use permanent generation when G1 is used as the garbage
collector. Here’s some clarification:

JDK 7: PermGen

Permanent Generation still exists in JDK 7 and its updates,
and is used by all the garbage collectors. In JDK7, the effort to remove the
permanent generation was started and some parts of the data residing in the
permanent generation were moved to either the Java Heap or to the native heap.
Permanent generation was not completely removed and it still exists in JDK 7
and its updates. Here's the list of things that were moved out of the permanent
generation in JDK7:

  • Symbols were moved to the native heap
  • Interned strings were moved to the Java Heap
  • Class statics were moved to the Java Heap

JDK7: G1 and PermGen

With G1 collector, PermGen is collected only at a Full GC
which is a stop-the-world (STW) GC. If
G1 is running optimally then it does not do Full GCs. G1 invokes the Full GCs
only when the PermGen is full or when the application does allocations faster
than G1 can concurrently collect garbage.

With CMS garbage collector, we can use option
-XX:+CMSClassUnloadingEnabled to collect PermGen space in the CMS concurrent
cycle. There is no equivalent option for G1. G1 only collects PermGen during the Full stop-the-world GCs.

We can use options PermSize and MaxPermSize to tune the
PermGen space size according to the application needs.

JDK8: PermGen

Permanent generation has been completely removed in JDK 8.
This work has been done under the bug
Options PermSize and
MaxPermSize have also been removed in JDK 8.

Email to openjdk alias regarding the PermGen elimination

JDK8: Metaspace

In JDK 8, classes metadata is now stored in the native heap
and this space is called Metaspace. There are some new flags added for
Metaspace in JDK 8:

  • -XX:MetaspaceSize=
    where is the initial amount of space(the initial
    high-water-mark) allocated for class metadata (in bytes) that may induce a
    garbage collection to unload classes. The amount is approximate. After the
    high-water-mark is first reached, the next high-water-mark is managed by
    the garbage collector
  • -XX:MaxMetaspaceSize=
    where is the maximum amount of space to be allocated for class
    metadata (in bytes). This flag can be used to limit the amount of space
    allocated for class metadata. This value is approximate. By default there
    is no limit set.
  • -XX:MinMetaspaceFreeRatio=
    where is the minimum percentage of class metadata capacity
    free after a GC to avoid an increase in the amount of space
    (high-water-mark) allocated for class metadata that will induce a garbage
  • -XX:MaxMetaspaceFreeRatio=
    where is the maximum percentage of class metadata capacity
    free after a GC to avoid a reduction in the amount of space